The electronics industry is entering into a heat constrained period of growth. The heat flux of electronic components is increasing and air cooling will no longer remove enough heat to maintain the desired operating temperatures of the microprocessors and other electronic components.
The maximum heat flux which is generally considered to be manageable by conventional air cooling is about 50 W/cm2. As microprocessors and other electronic devices are developed which create a heat flux in excess of about 50 W/cm2 the electronics industry is moving to liquid cooled heat sinks. One approach to such liquid cooled heat sinks is what is referred to as a microchannel heat sink. A microchannel heat sink has extremely small grooves formed in the material from which the heat sink is constructed so as to provide very thin fins separated by very thin microchannels. This provides a much larger surface area for the dissipation of heat. Combined with a forced liquid circulation system microchannel heat sinks provide one of the most promising solutions to the electronics industry's appetite for increased cooling capacity.
To date, microchannel heat sinks have been constructed from materials such as silicon, diamond, aluminum and copper, copper-tungsten composites, and ceramics such as beryllium oxide.
U.S. Pat. No. 5,099,311 to Bonde et al., the details of which are incorporated herein by reference, discloses a typical construction for a silicon microchannel heat sink including systems for delivery of coolant to the microchannels.
U.S. Pat. No. 5,099,910 to Walpole et al., the details of which are incorporated herein by reference, discloses a microchannel heat sink having U-shaped microchannels so that the direction of fluid flow alternates in adjacent microchannels so as to provide a more uniform temperature and thermal resistance on the surface of the heat sink. The Walpole et al. heat sinks are manufactured from silicon, a copper-tungsten composite such as Thermcon® or a ceramic such as beryllium oxide.
U.S. Pat. Nos. 6,457,515 and 6,675,875 to Vafai et al., the details of which are incorporated herein by reference, disclose multi-layer microchannel heat sinks having fluid flow in opposite directions in adjacent layers, so as to eliminate the temperature gradient in the direction of fluid flow across the heat sink.
U.S. Pat. No. 5,874,775 to Shiomi et al., the details of which are incorporated herein by reference, discloses a diamond heat sink.
U.S. Patent Publication No. 2003/0062149 to Goodson et al., the details of which are incorporated herein by reference, describes an electroosmotic microchannel cooling system.
There is a continuing need for improved materials for use in microchannel heat sinks to avoid some of the problems encountered with previously used materials. As will be recognized, the material from which the heat sink is formed (i.e., the substrate material) must be dimensionally stable so as to maintain its shape, yet it must also be leak-free, so the liquid coolant, under pressure, does not leak.
A dimensionally stable, leak-free graphite substrate can also be used in the formation of components for other applications, such as the flow field plates of electrochemical fuel cells. An electrochemical fuel cell such as an ion exchange membrane fuel cell, more specifically a proton exchange membrane (PEM) fuel cell, produces electricity through the chemical reaction of hydrogen and oxygen in the air. Within the fuel cell, electrodes, denoted as anode and cathode, surround a polymer electrolyte to form what is generally referred to as a membrane electrode assembly, or MEA. A catalyst material stimulates hydrogen molecules to split into hydrogen atoms and then, at the membrane, the atoms each split into a proton and an electron. The electrons are utilized as electrical energy. The protons migrate through the electrolyte and combine with oxygen and electrons to form water.
The MEA of a PEM fuel cell is sandwiched between two flow field plates. In operation, hydrogen flows through channels in one of the flow field plates to the anode, where the catalyst promotes its separation into hydrogen atoms and thereafter into protons that pass through the membrane and electrons that flow through an external load. Air flows through the channels in the other flow field plate to the cathode, where the oxygen in the air is separated into oxygen atoms, which join with the protons through the proton exchange membrane and the electrons through the circuit, and combine to form water. Since the membrane is an insulator, the electrons travel through an external circuit in which the electricity is utilized, and join with protons at the cathode.
The flow field plates are generally part of the structural support for the fuel cell, and thus have to be dimensionally stable. Also, since fluids like hydrogen, air and water flow about the channels formed along the surfaces of the plates, the plates need to be leak-free to avoid contamination or loss of reactants. Indeed, in some configurations, a cooling fluid flows between adjacent flow field plates, in which case having the plates leak-free is especially important.
Resin systems have been developed for the formation of dimensionally stable graphite sheets; resin systems have also been developed for the formation of leak-free graphite sheets. However, to date the production of graphite sheets, especially on which a pattern has been formed, which are both dimensionally stable and leak-free has not been possible. The formation of dimensionally stable sheets have led the formation of leak-paths through the sheets, while leak-free sheets have tended to be brittle and are therefore not dimensionally stable.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size, the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional such as thermal and electrical conductivity.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the “c” axis or direction and the “a” axes or directions. For simplicity, the “c” axis or direction may be considered as the direction perpendicular to the carbon layers. The “a” axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the “c” direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the “c” direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or “c” direction dimension which is as much as about 80 or more times the original “c” direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to as “flexible graphite”). The formation of graphite particles which have been expanded to have a final thickness or “c” dimension which is as much as about 80 times or more the original “c” direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal conductivity due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from high compression, making it especially useful in heat spreading applications. Sheet material thus produced has excellent flexibility, good strength and a high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a “c” direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 0.04 g/cc to about 2.0 g/cc.
The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon compression of the sheet material to increase orientation. In compressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the “c” direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the “a” directions and the thermal and electrical properties of the sheet are very different, by orders of magnitude, for the “c” and “a” directions.
Accordingly, what is desired is a substrate for the formation of electronic thermal management articles, components of electrochemical fuel cells, or the like, where the substrate is formed of one or more sheets of compressed particles of exfoliated graphite, and which is both leak-free and dimensionally stable and yet is capable of having a pattern or structure formed thereon.